1. Technical Field
The present invention relates to hydrodynamic pressure bearings, to spindle motors in which such bearings are utilized, and to disk-drive devices furnished with such spindle motors. The invention in particular relates to hydrodynamic pressure bearings capable of handling high rpm, spindle motors in which they are utilized, and disk-drive devices furnished with the spindle motors.
2. Description of the Related Art
The rotor in a spindle motor is rotatively supported along its axis by bearing means in a configuration such as disclosed for example in U.S. Pat. No. 5,558,445. This conventional spindle motor comprises a pair of thrust plates disposed on the upper and lower portions of a stationary shaft in its axial direction. A radial bearing section located between the pair of thrust plates is constituted by dynamic-pressure-generating striations formed between the outer circumferential surface of the stationary shaft and, opposing the outer circumferential surface in the radial direction, the inner circumferential surface of the rotor, and by lubricating oil retained therein. Under rotation of the rotor the dynamic-pressure-generating striations urge the lubricating oil in a predetermined direction to generate bearing pressure for supporting load on the rotor in the radial direction. Likewise, a pair of thrust bearing sections is constituted by dynamic-pressure-generating striations formed between the mutually opposing axially inward faces of each thrust plate, and the rotor axial faces opposing them in the axial direction, and by lubricating oil retained therein. Under rotation of the rotor the dynamic-pressure-generating striations urge the lubricating oil in a predetermined direction to generate bearing pressure for supporting load on the rotor in the thrust direction. The conventional configuration of a spindle-motor rotor bearing means meanwhile comprises first and second taper-seal sections for preventing the lubricating oil from leaking out to the bearing exterior. The first and second taper seal sections are configured in between seal caps opposing the thrust plates, and the rotor where it opposes the thrust plates, to vary in both radial and axial clearance.
Advantages to a configuration of this sort are that with the structure of the bearing sections being symmetrical, it imparts identical characteristics to, and therefore stabilizes, the rotation of the spindle motor no matter what its orientation, and that it enhances the component-material processing yield rate.
In the oil sealing structure in the foregoing conventional configuration, the first and second taper seal sections are ranged continuously. In this configuration, the position of the oil interface (gas-liquid interface) under ordinary conditions is positioned within the first taper seal section—where the clearance dimension varies in terms of the radial gap formed in between the outer circumferential surface of the thrust plate and the inner circumferential surface of the rotor; and under the action of centrifugal force during routine rotation, the position of the oil interface is made to encroach into the second taper seal section—where the clearance dimension varies in terms of the axial gap formed in between the upper face of the thrust plates and the lower face of the seal caps. This movement of the oil interface enlarges the capacity of the seal sections; at the same time, the taper-seal configuration directs the oil interface radially inward, exploiting centrifugal force to press the oil interface heading toward the thrust bearing sections and prevent the oil from flowing out.
The clearance dimension of the taper seal sections in terms of the gaps formed within the seal sections gradually expands parting off from the bearing sections, which produces a disparity in capillary force on the oil boundaries according to the position where each forms. If the amount of oil retained in the bearing sections has decreased, this disparity in capillary force acts to supply oil from the taper seals to the bearing sections. Likewise, if due to temperature elevation or the like the volume of oil retained within the bearing sections has increased, the taper seal sections function to accommodate that increase.
In applications in which the rotational speed of the spindle motor is sped still higher the influence of centrifugal force on the oil grows stronger, consequently enlarging the amount of oil flowing from the first taper seal section into the second taper seal section. Meanwhile, owing to dimensional constraints the second taper seals have in some cases been furnished orthogonal to the rotational center axis of the spindle motor. Lack thus of sufficient capacity secured to accommodate the greater amount of oil flow at higher rpm will be cause for concern that oil will flow out to the bearing exterior.
Another consequence of the amplified influence of centrifugal force due to heightened-speed rotation is that oil within the first taper seal section is torn off the outer circumferential surface of the thrust plate and becomes stuck on the rotor inner circumferential surface.
Within the thrust bearing section during high-speed rotation oil migrates radially outward under centrifugal force, bringing the retained amount of oil into a lowered state. The taper seal sections should then serve, as noted above, in a capacity of supplying oil to the bearing sections when the amount of oil retained therein has decreased; but oil becoming stuck on the inner circumferential surface of the rotor by the action of centrifugal force spoils the continuity of the oil, meaning that the supply of oil to the bearing sections under high rpm conditions will become inadequate. Depletion in the amount of oil retained in the thrust bearing sections impairs the bearing rigidity, which not only destabilizes rotational support, but also gives rise to contact and slippage between the thrust plate and the rotor occurs, producing seizure.